These are one of the best things I have discovered in Korea. Either use a Korean credit card or:

Go to one of those newspaper stands that has a sign in Korean which says: Gyo-tong cardu. Pay 12,000 for the card. You get 10,000 work of credit, but you save 8% on every ride. Recharge the card at the same place, a bank or at a subway station. You cannot buy the card at a subway station (of course!), but you may be able to buy one at a bank.

If anyone lives near Kangnam Station, PM me and I'll help you buy one.